Since the advent of the global positioning system (GPS), considerable research has gone into developing techniques for improving its performance. In particular, the technique of differential GPS (DGPS) is based on the fact that many errors in GPS measurements are relatively constant over short distances and can be eliminated by taking the difference between two nearby measurements. Consequently, DGPS provides improved position accuracy. For example, FIG. 1 shows a vehicle 12 located in the vicinity of a GPS reference station 14. A GPS signal measured by the GPS reference station can be transmitted to the vehicle where it is subtracted from a GPS signal measured by the vehicle to obtain a differential GPS signal with respect to a single satellite 16. Using several similar differential GPS signals from other satellites, together with the known location of the reference station, the vehicle navigational equipment can determine the vehicle location.
Traditional DGPS systems are based on the measurement of satellite pseudorandom noise clear acquisition (C/A) code signal phases. Although this approach can provide position determinations accurate to within a few meters, some navigational applications of GPS require accuracies on the centimeter scale. In addition, the integrity of this approach is also insufficient for some applications. For example, traditional DGPS does not meet all the specifications for a Category III (zero visibility) aircraft landing system.
One technique for improving the performance of traditional DGPS systems is called kinematic DGPS, or differential carrier phase GPS. Kinematic DGPS is based on carrier phase measurements rather than C/A code phase measurements. Because the GPS signal carrier has a wavelength of about 19 cm, these measurements provide a potential accuracy on the order of a centimeter. In order to use carrier phase measurements, however, it is necessary to accurately resolve the integer cycle ambiguity of the carrier signal. In many applications it is also necessary to resolve this ambiguity quickly.
An integrity beacon landing system (IBLS) developed by researchers at Stanford University resolves integer cycle ambiguities through the use of ground-based GPS transmitters called pseudolites 18. The pseudolites, which are placed beneath the final approach path of the aircraft 12, transmit low power GPS signals which define hemispherical bubbles through which the aircraft passes. As the aircraft passes through the bubbles, the geometry of the combined satellite/pseudolite constellation changes rapidly, which allows the integer cycle ambiguities from the satellites to be resolved with high accuracy and integrity. Once the ambiguities are resolved, centimeter level accuracy can be obtained, even after the aircraft exits the integrity beacon bubbles.
Although the IBLS system succeeds in providing integer cycle ambiguity resolution with high integrity and accuracy, it does not solve other problems inherent in all differential carrier phase GPS systems. In particular, it does not overcome the differential correction latency problem, that is, the delay between the time a measurement is taken by the GPS reference station 14 and the time the corresponding correction is received at the vehicle 12. The amount of the delay is determined by the data uplink frequency and by the time required for the link signal to travel from the reference station 14 to the vehicle 12. Because the delayed reference signal must be compared to a vehicle signal which was measured at the same time, the calculated vehicle position will correspond to a past vehicle position rather than the present vehicle position. Consequently, the latency introduces undesired uncertainty in the present vehicle position.
Conventional kinematic DGPS systems either tolerate the position uncertainties introduced by differential correction latency, or use inertial integration techniques to determine the present vehicle position from the calculated past position and additional inertial data. In many automated vehicular control applications, such as Category III landing systems, the latency introduces an intolerable degradation in control performance. Inertial integration techniques, on the other hand, have the disadvantage that they introduce considerable hardware and software complexity to the navigational system.